PURPOSE: To prevent the transmission of the insignificant polling commands of the terminals in a polling control system if a terminal that is not packaged yet or has an unapplied power supply is mixed on a circuit.

CONSTITUTION: A partner carrier monitoring part 1-1 monitors whether the transmission of a carrier should be started or not by the pertner terminal for transmission of an EOT code or a telegram in an answer reception waiting state set after a polling command is transmitted. When the polling command receives no answer, it is judged whether this fault is caused by a terminal that is not packaged or has an unapplied power supply or caused a temporary transmission fault based on the carrier state of the partner terminal. Then a polling control part 1-2 inhibits the retransmission of the insignificant polling commands to the unpackaged terminal.